cat conCATenates one or more files given as argument and prints those on the standard output (console).
If no file or ‘-‘ (dash) is given, it reads from standard input until EOF (Ctrl+D) is pressed and prints is to standard output.

$ ls -l linus # ls to make sure file exists
-rw-rw-r-- 1 daniel daniel 403 Apr 15 01:20 linus
$ cat linus # print content of file to stdout
Hello everybody out there using minix -
I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones.
This has been brewing since april, and is starting to get ready.
I'd like any feedback on things people like/dislike in minix,
as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things).
$ cat -n linus # show line numbers with -n
1 Hello everybody out there using minix -
2
3 I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones.
4 This has been brewing since april, and is starting to get ready.
5 I'd like any feedback on things people like/dislike in minix,
6 as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things).
$ cat # read from stdin(keyboard) and print to stdout(screen), repeats after me until I press Ctr+D to end it.
reading
reading
from standard inpu
from standard inpu
pressing Ctr+D now
pressing Ctr+D now
$ cat << EOF > file-from-stdin # Reads from keyboard until EOF is pressed and saves(redirects) the text to a file.
I read this from stdin
end of file
EOF
$ ls -l file-from-stdin # printing content of file we created above.
-rw-rw-r-- 1 daniel daniel 35 Apr 15 01:22 file-from-stdin
$ cat file-from-stdin
I read this from stdin
end of file

4. clear – clears the terminal screen.

clear is self-explanatory, it clears the terminal display and places your cursor at the top left corner. Similar to “cls” command in DOS/Windows/PowerShell.

Use rmdir to delete the named directory, not its contents. To completely wipe out a directory and its contents use ‘rm -r’.

6. mkdir – make or create one or more directories.

mkdir is used to create one or more directories under current directory if no directory argument is given. The user creating the directory must have the permission to create a directory under the specified directory.

While connected to a remote host through an ssh connection using Putty or other ssh clients, your client might freeze and not respond to any keyboard activity. In order to force an exit, there is a “secret” keyboard shortcut – Enter~.[ Enter followed by ~ followed by .(dot) ]

ipython is program which allows you to run python code in an interactive shell. Although Python itself when run from CLI opens an interactive shell as well, ipython is much more powerful and greatly improves your productivity. Some of the things you can do with ipython but not the default python shell is command or code and file name completion, view history, copy/paste a single or multiline code, nicely colored help with in the shell, run Linux commands such as ls or cat, scroll up/down to previous commands, automatically adds spaces after you press enter, etc.

Deleting sensitive data from history
You can delete any line from history by using SQL. First use SELECT statement to find the line number and then use DELETE statement to delete it. In this example, we are deleting line number 10 from the history table –

Problem statement – You have a file with hundreds or thousands of lines and you want to copy the contents of this file and paste it to an external application, for instance to a browser.

Solution – The first attempt is to try to cat the file and scroll down with your mouse to select each line. This is time consuming or in some cases might not work if there are too many lines as some of the lines will ‘scroll out of the terminal’. One way of getting around this is to use “xclip” – a command line interface to X selections (clipboard).

In my case I wanted to copy the contents of ‘/tmp/ipaddresses.txt’ file to a browser for blogging. The file had 10000 lines. I used the following commands, first to install xclip and then to copy the file contents to a clipboard –

apt-get -y install xclip
xclip -sel cli < /tmp/ipaddresses.txt

The xclip command basically does a selection (-sel) from the file into the clipboard(-cli), where you can copy paste to any other external application.

You can also use the ldd command to find out if an executable has an expected dependencies. In this case, we expect that the htpasswd, login and sshd commands depend on the crypt library as they prompt a user for a password for authentication purposes –

1. With the chage command, an account expiration can be set. Once that date is reached, the user cannot log into the system interactively.
Let us run ‘chage’ interactively to set user’s account expiry –

2. In addition to this, the usermod command can “lock” an account with the -L option. Say when a user is no longer with a company, the administrator may lock and expire an account with a single usermod command. The date must be given as the number of days since January 1, 1970. Setting the expiredate to 1 will immediately lock the account –

Locking the account prevents the user from authenticating with a password to the system. It is the recommended method of preventing access to an account by an employee who has left the company. If the employee returns, the account can later be unlocked with

usermod -U USERNAME

. If the account was also expired, be sure to also change the expiration date.

3. The nologin shell. Sometimes a user needs an account with a password to authenticate to a system, but does not need an interactive shell on the system.
For example, a mail server may require an account to store mail and a password for the user to authenticate with a mail client used to retrieve mail.
That user does not need to log directly into the system.

A common solution to this situation is to set the user’s login shell to /sbin/nologin. If the user attempts to log into the system directly,
the nologin “shell” will simply close the connection.

When we think of running scheduled tasks in Linux, the first tool which comes to mind to most Linux users and admins is cron. Cron is very popular and useful when you want to run a task regularly – say after a given interval, hourly, weekly or even every time the system reboots. The scheduled tasks are faithfully executed by the crond daemon based on the scheduling we set, if somehow crond missed the task because the machine was not running 24/7, then anacron takes care of it. My topic today though is about at which executes a scheduled task only ones at a later time.

1. Adding future commands interactively –

Let us schedule to run a specific command 10 minutes from now, press CTRL+D once you have entered the command –

In this small tutorial about at utility, we saw some of the use cases for at – especially where we had to execute a scheduled task only once. The time specification it uses is human friendly, example it supports time specs such as midnight, noon, teatime or today. Feel free to read the man pages for details.

You might have accidentally deleted a configuration or binary file which was installed as part of a package OR may be you modified the original file and you want to restore the original as you didn’t take a back – this blog will help you in resolving similar issues.

The steps below are for Redhat/CentOS based Linux systems, where the package was installed using rpm or yum. The steps basically outline how to grab the rpm package, unpack and gain access to the files inside the rpm. I will demo the steps i used to recover ntp.conf –

Tcpdump is a handy tool for capturing network packets. It will keep on capturing packets until it receives a SIGINT or SIGTERM signal, or the specified number of packets have been processed. If you have tried to pipe the output of tcpdump to a file or tried to grep it, you will notice a significant delay before you even see an output. The reason behind that is, tcpdump buffers output in 4k byte chunks and it doesn’t flush it until 4k of data is captured.

To get around the buffering, you can use ‘-l’ option to see the packets captured in real time in order to ‘grep’ or ‘tee’ output to a file. From the man page –

Per the man page, the yum-config-manager is “a program that can manage main yum configuration options, toggle which repositories are enabled or disabled, and add new repositories.” The details on how to use the command is in the Official Redhat documentation.

One feature that the man page does not list is how you can use the yum-config-manager to display the yum repo configuration sections/directives and options. Not only can you use it to just show the configuration in your system, but it can also help you with displaying all the options supported by yum configuration. It might be useful for scripting as well.

Installation – identify the package name:

yum whatprovides */yum-config-manager

Install package –

yum install yum-utils

Once the package is installed, the command yum-config-manager should be available –